An imaging bar code scanner operates in environments where it is constantly exposed to dirt, debris, and other contaminants. Some of this dirt, etc., will get deposited on one or more of the scanner's optical windows, and will necessarily degrade the quality of images acquired by that scanner. Degraded image quality, in turn, naturally degrades performance of the scanning equipment.
In an imaging bar code scanner, light traverses one or more optical components on its way from the imaged object to one or more image sensors. Optical components perform at their best when they are free of dirt, grease, and any other contaminants that may scatter, absorb, or diffract light as it travels from the object to the sensor. Typical operations of a bar code scanner in a Point-Of-Sale (POS) system constantly expose parts of the scanner to dirt, etc., from the environment, some of which may be deposited on one or more of the scanner's optical components. The overall effect of the dirt then is that it may obscure the image in the places where the dirt is interposed between the object and the sensor. An additional complicating factor is that the dirt may appear different to the image sensor, depending on whether the dirt is back-lit by illumination from the environment, or whether the dirt is front-lit by illumination coming from the scanner itself. In both cases the dirt decreases the ability of a bar code scanner to perform its primary function, which relies upon the acquisition of images that are essentially free from optical defects, including defects caused by dirt, etc., in the optical path.
It is therefore desirable to have an imaging bar code scanner that can detect when dirt is obscuring the image. It is also desirable to have an imaging bar code scanner that can alert an operator that the scanner's optical components need cleaning. Furthermore, it is desirable to have a scanner that can detect when it is clean again so that the operator alert can be set to a non-alerting state. Finally, it is desirable to have a scanner that is equally effective at detecting both back-lit and front-lit dirt.